1. Field of the Invention
The present invention relates to a carbon capsule with a hollow core and a mesoporous shell (hereinafter, referred to as “HCMS”) structure, a catalyst as a fuel electrode (anode) or an oxygen electrode (cathode) of a PEM (polymer electrolyte membrane) fuel cell employing the HCMS carbon capsule as a support material, and a method of preparing the catalyst, and more particularly, to an HCMS carbon capsule having high activity against oxidation reaction of fuel such as methanol, ethanol or hydrogen and against oxygen reduction reaction in a PEM fuel cell including a direct alcohol fuel cell such as DMFC (direct methanol fuel cell), a DEFC (direct ethanol fuel cell) or the like, and a hydrogen fuel cell, a catalyst employing the HCMS carbon capsule as a support, and a method of preparing the catalyst.
2. Discussion of Related Art
A fuel cell is a device directly transforming chemical energy of fuel into electric energy and heat energy through electrochemical reactions. In the fuel cell, there is no combustion process and there is no driving unit, so that energy transformation efficiency is high and environmental problems do not arise. For this reason, researches on the fuel cell have been being carried out in many countries to be used in various applications such as power generation, household power, carriage power, mobile power, etc. since it is developed as a power source for a spacecraft based on a space program of America in the 1960s. As the fuel cell, a direct alcohol fuel cell directly employs methanol or ethanol as fuel, and liquid methanol or liquid ethanol is directly supplied to a fuel electrode (anode), so that the fuel cell can be driven at a relatively low temperature and have a small size, thereby attracting attention as next-generation alternative energy for small mobile power, automobile power, etc. On the other hand, a hydrogen fuel cell attracts attention as an energy source for an automobile, a carriage or the like requiring relatively higher power.
Generally, the cathode and anode electrodes used in the fuel cell are prepared by loading a catalytic metal such as Pt, Pt—Ru alloy or other various active metal alloys in a carbon support.
For example, in the case of an over-the-counter direct methanol or ethanol fuel cell, a fuel electrode thereof employs a Pt—Ru catalyst (e.g., E-TEK Inc.) including catalytic metal such as 60 wt. % Pt—Ru (molar ratio 1:1) loaded in a carbon support such as carbon black (Vulcan XC-72) having a high surface area, or employs Pt—Ru (molar ratio of 1:1) catalytic metal itself without the carbon support. Further, in the case of the over-the-counter direct methanol or ethanol fuel cell, an air electrode thereof generally employs a Pt black catalyst. Generally, as the fuel electrode of the direct alcohol fuel cell, the catalytic metal loaded in the carbon support has excellent activity as compared with the non-loaded catalytic metal. Further, the larger the surface area and the pore volume of the support are, the smaller the size of the catalytic metal to be loaded in the support is. Therefore, in the case where the surface area and the pore volume of the support are large, the catalytic metal can be uniformly distributed in the support, so that the active area thereof is increased and thus the fuel and a reaction product are readily dispersed, thereby enhancing the activity.
Recently, the electrocatalyst employed for oxidation-reduction reaction of the fuel and air in the fuel cell has been prepared by distributing fine particles of the catalytic metal over the support having high conductivity and high surface area such as the carbon black having a surface area of about 230 m2/g (e.g., Vulcan XC-72 of Cabot Co.). As for enhancing the activity of the fuel cell, the electrocatalysts such as Pt—Ru/C, Pt/C, etc. should satisfy the following characteristics.
First, the particles of the catalytic metal should be small so as to be uniformly distributed on the support, so that the area of an active site at which catalysis reaction is performed is increased, thereby increasing the catalyst activity with respect to the oxidation-reduction reaction.
Second, carbon used for the support should have a porous structure through which gas and water smoothly pass and with which the particles of the catalytic metal are loaded strongly enough to prevent the loaded particles of the catalytic metal from easily separating due to physical and chemical impacts.
Like this, the support used in the electrode of the fuel cells is in need of an optimum porous structure, an excellent electrical conductivity, a high surface area, and large pore volume. Particularly, in the fuel electrode of the direct alcohol fuel cell, the fuel including the methanol and the ethanol mixed with water passes through an electrocatalyst support and is oxidized in a catalyst layer, and thus the electrocatalyst support should be excellent in electrical conductivity and hydrophilicity and should have a uniformly sized and three-dimensionally developed internal porous structure through which the fuel and the gas can smoothly pass. Accordingly, to provide a method of preparing porous carbon excellent in the conductivity and having both the uniform pores and the high surface area, the present inventors have filed Korean Patent Application No. 2000-57082, titled “a method of preparing a porous carbon molecular sieve having a regular size using a liquid carbon precursor”, and Korean Patent Application No.2002-8376, titled “a method of preparing a nano capsule structure with a hollowcore and a mesoporous shell”, which will be incorporated in the present application.
However, there has not been developed the electrocatalyst supported by the HCMS carbon capsule. Accordingly, research on the electrocatalyst supported by the HCMS carbon capsule is made to enhance the activity of the conventional electrocatalyst of the fuel cell employing the carbon black as the support.
Thus, the present inventors tested the activity of the electrocatalyst prepared by using the HCMS carbon capsule as the support for the cathode and the anode of the PEM fuel cell such as the direct alcohol fuel cell and the hydrogen fuel cell. As a result of the test, the present inventors found out that the electrocatalyst using the HCMS carbon capsule has excellent activity as compared with the conventional catalyst.